Dynamic random access memory (DRAM) devices provide a large system memory and are relatively inexpensive because, in pan, as compared to other memory technologies, a typical single DRAM cell consists only of two components: an access transistor and a capacitor. As is well known in the art, the storage capability of the DRAM cell is transitory in nature because the charge stored on the capacitor leaks. The charge can leak, for example, across the plates of the capacitor or out of the capacitor through the access transistor. As a result, DRAM cells must be refreshed many times per second to preserve the stored data. With the refresh process being repeated many times per second, an appreciable quantity of power is consumed. In portable systems, obtaining the longest life out of the smallest possible battery is a crucial concern, and, therefore, reducing the need to refresh memory cells and, hence, reducing power consumption is highly desirable.
The refresh time of a memory cell is degraded by two major types of leakage current; junction leakage current caused by defects at the junction boundary of the transistor and channel leakage current caused by sub-threshold current flowing through the transistor. Leakage current can be reduced by increasing the magnitude of the gate-to-source voltage that is applied to turn OFF the access transistor and leaving the threshold voltage of the transistor the same. Thus, instead of applying zero volts on the word line to turn OFF an NMOS access transistor, a negative voltage of −0.3 volts may be applied to the word line, decreasing the transistor's current leakage for a given threshold voltage.
The application of a negative voltage to the word line must be precisely controlled or the channel of the pass gate which isolates the storage capacitor may be significantly stressed or even damaged. Therefore, a stable and accurate voltage reference has been conventionally employed for generating a negative voltage word line (VNWL) signal. Desirably, precision voltage references should be insensitive to manufacturing (process) and environmental variations, voltage variations, and temperature variations (PVT variations).
One of the more popular voltage reference generators for generating a negative voltage reference signal for coupling to the inactive word lines includes a bandgap voltage reference. Typically, a bandgap voltage reference circuit uses the negative temperature coefficient of emitter-base voltage differential of two transistors operating at different current densities to make a zero temperature coefficient reference. Such an approach proved adequate until advances in sub-micron CMOS processes resulted in supply voltages being scaled-down with the present processes operating at sub 1 volt supply voltages. This trend presents a greater challenge in designing bandgap reference circuits which can operate at very low voltages. Even though conventional bandgap circuits can generate a PVT insensitive voltage, the minimum supply voltage Vcc required for proper operation at cold temperatures is approximately 1.05 V.
FIG. 1 illustrates a conventional circuit diagram of a voltage reference generator 10 including a bandgap voltage reference 12 configured to generate a signal VBG1. The bandgap voltage reference 12 includes a divider network including a resistive (L*R) element 20 and a diode (1X) element 22 coupled to a first input of a differential amplifier 18. A second input of the differential amplifier 18 is coupled to a divider network including a resistive (L*R) element 24, resistive (R) element 26 and a diode array (8X) element 28. Signal VBG1 couples to a differential amplifier 30 and generates a reference signal 32. In the conventional voltage reference generator 10, bandgap voltage reference 12 outputs signal VBG1 with a potential of approximately 1.2 volts to 1.3 volts. Signal VBG1 goes through differential amplifier 30 to generate reference signal 32 having a potential of approximately −0.3 volts. Signal VBG1 must be set at about 1.3 volts to get the zero temperature coefficient as shown by:(VBG1)=L*n*lnK*Vt+Vd1                 where, L is the resistor ratio, n is the process constant (approx.=1), K is the BJT ratio, Vt is the thermal voltage (about 25.6 mV at room temperature has a temperature coefficient of about 0.085 mV/C), and Vd1 is the voltage at the 1X diode (about 0.65 volts at 27° C. has temp. coefficient of about −2.2 mV/C).        In order to have a zero temperature coefficient, L*n*lnK*0.085 mV=2.2 mV, so the L*n*lnK must be about 2.2 mV/0.085 mV=25.8.        Thus, VBG1=25.8*25.6 mV+0.65=1.31 volts.Since signal VBG1 is about 1.3 volts, the minimum power supply voltage for the bandgap voltage reference circuit shown in FIG. 1 must be higher than 1.3 volts, which is unacceptable for circuits that operate on a supply voltage Vcc of less than 1.2 volts.        
FIG. 2 illustrates another conventional circuit diagram of a voltage reference generator 50 that includes a bandgap voltage reference 52, which is configured to generate a signal VBG2. Bandgap voltage reference 52 includes a network including a resistive element 60 and a diode (1X) element 62 coupled to a first input of a differential amplifier 58. A second input of the differential amplifier 58 is coupled to a network including a resistive element 64 and a diode array (8X) element 66. Signal VBG2 couples to a unity buffer 68 and a differential amplifier 70 and generates a reference signal 72. In the conventional voltage reference generator 50, the CTAT current flows through a PTAT resistor 74 to generate a zero temperature coefficient output signal VBG2 of about 0.6 volts. The voltage reference generator is then buffered and connected to the differential amplifier 70 to generate a −0.3 volt reference voltage. One disadvantage of this approach occurs during cold temperature operation when the voltage on the diode (1X) element 62 at the cold temperature becomes higher (e.g., about 0.82 volts at −40° C.). Accordingly, additional voltage (e.g. 0.2 volts to 0.3 volts) is needed for the PMOS devices in the amplifiers to remain in the saturation region. Thus, the minimum power supply voltage for the bandgap voltage reference 52 shown in FIG. 2 must be higher than 0.82 volts+0.23 volts=1.05 volts. Although bandgap voltage reference 52 may output a lower potential for signal VBG2 than the conventional bandgap voltage reference 12 of FIG. 1, the minimum acceptable supply voltage Vcc of the voltage reference generator 50 of FIG. 2 remains above 1.0 volt (e.g., 1.05 volts) which is unacceptable for circuits that desire to operate on a supply voltage Vcc of less than 1.0 volt.
There is a need for systems, devices, and methods for generating a low-voltage reference signal that remains relatively stable for a broader range of operating voltages including lower operating potentials.